Detection of melamine in the food has received much attention since several food safety incidents of melamine adulteration occurred in recent years. We previously developed a simple and rapid colorimetric method to detect melamine, which relies on melamine-induced aggregation of gold nanoparticles (GNPs). However, the mechanism behind this method is still not clear. Herein we systematically characterized the melamine-induced gold nanoparticle aggregates. Various experimental studies, including dynamic light scattering, zeta potential, UV-visible absorption spectroscopy and transmission electron microscopy (TEM), were carried out. The results have revealed that melamine is directly adsorbed to the GNP surface, thus decreasing the surface potential of GNPs in a concentration-dependent manner. Moreover, based on the understanding of the mechanism, we have been able to further improve our previously developed melamine detection method by introducing a critical coagulation concentration (CCC)-based salt titration step. This CCC-based salt titration not only significantly increases the detection sensitivity, but also enables a reliable quantification without any equipment.